This invention relates generally to vortex-shedding flowmeters, and more particularly to a flowmeter of this type which includes a torsionally mounted sensor and a balanced piezoelectric torque transducer associated therewith to provide an output signal whose frequency is proportional to the flow rate of the fluid being metered whereby the meter is useful for both liquid and gas flow rate measurement.
It is well known that under certain circumstances the presence of an obstacle or shedder in a flow conduit will give rise to a periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate.
It is this phenomenon which is exploited to create a flowmeter to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. Flowmeters of this type are disclosed in Bird U.S. Pat. No. 3,116,639, and in White U.S. Pat. No. 3,650,152. Existing flowmeters of the vortex shedding type, such as those disclosed in the Burgess U.S. Pat. No. 3,888,120 and the Herzl U.S. Pat. No. 4,162,238, are capable of effecting volumetric or mass flow measurement.
In my first copending patent application Ser. No. 944,624, filed Sept. 21, 1978, entitled "Vortex-Shedding Flowmeter Having a Sensing Vane," the entire disclosure of which is incorporated herein by reference, there is disclosed a vortex-type flowmeter in which fluidic oscillations produced by a shedder are sensed by a downstream balanced-vane sensor pivoted in a torsional suspension that allows only microscopic vane motion. The shedder mounted in the flow tube acts to divide the incoming fluid flowing therethrough and causes vortices to be shed alternately on either side thereof. The downstream train of vortices passing on either side of the vane sensor generates fluidic forces giving rise to alternate clockwise and counterclockwise torques, causing the sensor to oscillate mechanically at a frequency proportional to the flow rate of the fluid being metered.
My second copending patent application Ser. No. 013,557, filed Feb. 21, 1979, entitled "Vortex Shedding Flowmeter Having Drag-Actuated Torsional Sensor," is a continuation-in-part of the above-identified first application Ser. No. 944,624, the present application being a continuation-in-part of the Second application. In the second application, whose entire disclosure is incorporated herein by reference, torsionally-supported behind the shedder is a drag-actuated sensor which includes a pair of parallel legs symmetrically disposed with respect to the longitudinal axis of the flow tube.
With a drag-actuated sensor of this type, a vortices are successively detached from the shedder and appear alternately on either side of the gap between the shedder and the downstream sensor, the low pressure region generated by each vortex acts to displace the stagnant zone produced in this gap as a result of fluid flow past the shedder to a position in front of the adjacent leg of the sensor, the fluid flow then going around and past the other leg, thereby developing a torque about the pivot axis. These torques are developed alternately, causing the torsionally-supported sensor to oscillate at a frequency in accordance with flow rate.
In both the first and second patent applications, the oscillatory motion of the torsionally-supported sensor is detected by means of a transducer which takes the form of a strain gauge bonded to a resilient beam, one end of which is attached to the trunnion or shaft of the sensor projecting through the flow tube, the other end being anchored. The resultant deformation of the beam as the shaft oscillates is translated by the strain gauge into a corresponding electrical signal whose frequency is indicative of flow rate. These patent applications also note that in lieu of a strain gauge, one may detect the deformation of the resilient beam by a piezoelectric element of other forms of micro-motion detectors.
As pointed out in the first and second patent applications, an important advantage of a vortex flowmeter having a torsionally-mounted sensor is that the meter is effective and accurate for both liquid and gas flow measurements.
Though the vortex-type flowmeters disclosed in the first and second patent applications represent a significant advance over prior art vortex-type meters, such as those disclosed in the above-identified patents, their torque transducer arrangements have certain drawbacks and therefore fall short of an ideal arrangement.
Inasmuch as the torque transducer arrangement disclosed in the present application closely approaches the ideal requirements for a sensing system constituted by a torque transducer associated with a torsionally-mounted sensor in a vortex-type flowmeter, these ideals will now be set forth:
A. The system has a sensitivity which renders the meter effective for low-pressure gas measurement.
B. The system is one which has an inherent ruggedness that renders the meter suitable for heavy-duty liquid flow rate measurement.
C. The system is insensitive to mechanical vibration and shock and acceleration forces to which the flowmeter is subjected.
D. The system is capable of operating over the broad temperature range encountered in gas and liquid measurement and is capable of operating over a very wide operating frequency range.
E. The sensing system requires virtually no motion and is not limited by torque transducer bonding or attachment problems.
F. Finally, the sensing system is one which is relatively inexpensive and has a compact structure.